Billet Vs press-together crankshaft

[Billzilla]

I'm hoping that someone here can give some advice on press-together cranks, as can be found in some motorbikes. I'm looking at a flat-plane V8 crank.
I can see how there could be some distinct advantages over a billet crank, such as being able to have hollow big-end journals, which would allow me to reduce the counterweight masses, etc. With the ability of good CNC gear to make nearly anything and do it very well, I was also wondering if instead of the usual interference-fit parts that are cylindrical and are pressed together when the male part of cold and the female part is heated, then a touch of weld to lock them solidly, could you get those parts machined so that they have, say, an octagonal (or more sides, each blending into the next with a small radius) shape but still require a reasonable temperature differential to get them together? Still an interference fit otherwise though.

I'm also wondering about the overall strength; In a conventional billet crank the stress areas are naturally in the corners of the various bearing journals, but with a press-together type would those stress areas be much different as the corners are made from two parts instead of the one? Better or worse?

[Greg Locock]

Ultimately your octagon becomes a spline, so we know that works. St Venants principle says that the details of structure far away don't affect the stresses locally. In other words unless the press fit grossly affects the structure, the stresses at the corners are define by the loads from the conrod.

I'd have thought the many difficulties associated with a press together crank would make it the more exotic choice of the two, and i'm a conservative.

[Billzilla]

Thanks for the reply - Long time no chat!!
Yes that makes sense. The press-together cranks are still used in high-power bikes, including supercharged drag bikes, so I'm not particularly worried about it failing, just wondering if there would be any real difference in the strength compared to a billet crank. Your reply makes me think that it wouldn't be significantly different. For sure it's more exotic, but it would definitely let me build a lighter crank overall.

[Hoffman900]

Thanks for the reply - Long time no chat!!
Yes that makes sense. The press-together cranks are still used in high-power bikes, including supercharged drag bikes, so I'm not particularly worried about it failing, just wondering if there would be any real difference in the strength compared to a billet crank. Your reply makes me think that it wouldn't be significantly different. For sure it's more exotic, but it would definitely let me build a lighter crank overall.

Bill,

Are you sure about that? I know the Harley based Pro Stock stuff went to a non-presses billet crank.

When Indian built the FTR750 flat track engine, it also has a billet crank with split end connecting rods, to get away from the a non split crank.


I’m not aware of any Multi cylinder set up using a split crank, at least a very high output one. I’ll share more later.

[e36jon]

I am no expert in this area, but wanted to put some stuff out there to stimulate the conversation.

1. Big (Massive? Colossal?) marine engines often have built-up crankshafts. Not everything is relevant to what you are asking, but a lot is. Google is your friend.

2. If your driver for wanting to design and build this built-up crank is weight reduction there are often ways to get what you want via more conventional means. I haven't seen EDM used for machining of journal interiors, but for a single crankshaft, maybe it works? (Or Unicorns, or witchcraft, or whatever that will work for one or a few but not for many.) There are some great tech papers floating around with info on making race crankshafts...

3. There is a multi-lobe polygonal shaft interface that is an alternative to splines. It's designed to limit stress concentration, which may not be the main issue. Looking online just now there are a number of research papers on wear / strength issues of this interface relative to others.

4. Possibly not relevant at all, but, the built-up-camshaft world has some similar challenges and has been pretty documented...

[Billzilla]

I am no expert in this area, but wanted to put some stuff out there to stimulate the conversation.

1. Big (Massive? Colossal?) marine engines often have built-up crankshafts. Not everything is relevant to what you are asking, but a lot is. Google is your friend.

2. If your driver for wanting to design and build this built-up crank is weight reduction there are often ways to get what you want via more conventional means. I haven't seen EDM used for machining of journal interiors, but for a single crankshaft, maybe it works? (Or Unicorns, or witchcraft, or whatever that will work for one or a few but not for many.) There are some great tech papers floating around with info on making race crankshafts...

3. There is a multi-lobe polygonal shaft interface that is an alternative to splines. It's designed to limit stress concentration, which may not be the main issue. Looking online just now there are a number of research papers on wear / strength issues of this interface relative to others.

4. Possibly not relevant at all, but, the built-up-camshaft world has some similar challenges and has been pretty documented...

Thanks to everyone that replied.

e36jon - I'm interested in all your points, but especially in your 2nd & 3rd points. Got any links please?

[63l8qrrfy6]

I think both are bad if you don't do any strength calculations

Generally speaking the predominant load in a low speed supercharged application will be bending so the last thing you want is a stress raiser where the crank pin meets the cheek. In this particular instance you probably want a hirth joint in the middle of the pin which implies you'll have to use some form of rolling element bearing.
A high speed N/A engine crank going through torsional resonance will be limited by the torque capacity of the joint regardless of where that is on pin.
Why not use a billet crankshaft? If you are planning on using journal bearings there are many machining schemes that would allow you to hollow out the pin and it will be stronger for the same general dimensions.

Finally, on polygon interference profiles: no matter how precisely you machine the internal and external forms you will always end up with a significant profile deviation on both sides compared to a simple circular interference. As a result, the interference pressure will always have large variations across the profile which means localized fretting is very likely.
Finally, to interfere a polygon you have to do it on a taper such that the actual interference is controlled by the press distance.

[e36jon]

e36jon - I'm interested in all your points, but especially in your 2nd & 3rd points. Got any links please?

2. "Other ways to reduce crankshaft weight": Honda published this a while back, and in #8 they discuss using friction-welding to join sections: http://www.f1-forecast.com/pdf/F1-Files ... P2_08e.pdf I had forgotten this second example of using additive manufacturing (Also Honda.), https://www.autodesk.com/redshift/crankshaft-design/

3. "Multi-lobe polygonal interfaces": (I just googled the title at left. I have not read or internalized any of the linked documents. The examples given are just to show that there is a lot of info on this out there and you would not need to invent anything to use this interface.) https://pdfs.semanticscholar.org/140b/7 ... 509c75.pdf, https://www.gvsu.edu/cms4/asset/777A03C ... esis_2.pdf
The fretting paper(s) are behind pay-walls, but you will see them is you do a Google search.

I know Honda used friction-welding, but I wonder about other non-mechanical methods: Brazing? Electron beam welding? Adhesive(s) (OK, this one give me the creeps.), etc...

This link has a ton of relevant info, including a great example of a flat-plane crank that has drilled pins, with resultant crazy looking counterweights, and an example of a built-up crank from a radial engine: http://www.epi-eng.com/piston_engine_te ... issues.htm

Saw this Tatra V10 (& V8) diesel built-up crank (Others are also discussed in the thread.): https://www.thesamba.com/vw/forum/viewt ... ?p=3512182

Cheers,

Jon

[Billzilla]

Excellent information, many thanks gents!

[djos]

Interestingly, in the rotary world, the eccentric shafts for 3 and 4 rotor engines are Billett multi-piece units and not welded.

https://www.atkinsrotary.com/store/90-9 ... -D00C.html

[Tommy Cookers]

electron beam welding is great only if .... there's no .....
residual magnetism in the parts being welded (magnetism will bend the electron beam)
otherwise the 'weld' zone won't be where it was intended to be ie the weld will be partly or totally 'missing'

so before welding check/demagnetise as necessary and ....
after welding inspect weld internally eg ultrasonically
external inspection alone is inadequate

I temporarily became a minor hero for this bit of detective work (with maraging steel)

[humble sabot]

From a cursory reading of the responses here and my own years of developed intuition, as far as the crankshaft itself is concerned, it's a matter of how you do the math on six of one and half a dozen of the other. Any time you add an interface you add weight in that location when compared to a monolithic version, so bulk of the benefit you'll see is in the mass of the big end journal itself. If assembled the hard way, i could see making one piece rods part of the equation, that might help you with big end mass. But it means you'd need to dismantle the crankshaft to replace a worn/bent/broken rod.
I probably wouldn't bother either way as i expect you'll pay a windage penalty too. Depending on how you split the elements the oil flow within the crank could be easier or more difficult to run than a normal cross drilled solution without inducing cavitation.

[Billzilla]

Thanks for extra information.
A billet crank would seem to be the preferred option, with a bit of more exotic machining to reduce the big-end mass. I think I've worked out a way to get good oil feed, even with a large hole in the big-ends of the crank.

[63l8qrrfy6]

Post CAD so we can give you more confusing (and conflicting) advice!

[e36jon]

I can't remember if anyone shared the link to Brians' excellent Cosworth TJ crankshaft article: viewtopic.php?p=819855#p819855

I think you'll appreciate how oil gets from point A to point B... I loved that they had add back plugs to the big end drilling to create oil galleries. I know a guy (Wink wink... No association here, just a fan.) with a set of the plugs for sale: https://www.racetothefinish.co.uk/mecha ... ainer-set/ Reduce. Reuse. Recycle. AmIright?



Jon